"What we are trying to do is follow a two-track strategy," Kurt Campbell, assistant secretary of state for East Asia and Pacific affairs, told reporters on arrival in South Korea from Japan.

"Under the right circumstances, we'd be prepared to sit down with North Korea if they would abandon their nuclear ambitions," Yonhap news agency quoted him as saying.

"We're in the process of discussions with our partners about what are the next steps associated with diplomacy."

After the United Nations Security Council censured its April 5 long-range rocket launch, the North announced it was quitting six-party nuclear disarmament talks with the United States, South Korea, China, Russia, and Japan.

It staged its second nuclear test on May 25, prompting the Council to adopt a resolution imposing tougher sanctions.

On Thursday the Council imposed a travel ban on five North Korean officials and asset freezes on five more entities involved in missile or nuclear programmes.

Campbell, who is making his first trip to the region since taking over from Christopher Hill last month, travels on to Thailand next week for the ASEAN Regional Forum on security issues.

Following a meeting with Deputy South Korean Foreign Minister Lee Yong-Joon, Campbell urged the North to return to the six-party talks, warning that the impoverished communist state would face more isolation and hardship.

"Truth of the matter is, down this path North Korea has chosen lie greater tensions, greater hardships for its people, more isolation and lack of engagement in international economy," he told journalists.

"I think it's unsustainable, and we believe that over time, North Korea will ultimately choose to re-engage," he said.

During an hour-long meeting with Lee, the two talked about the need for the two allies to stand together as they confront "very difficult challenges" on the Korean peninsula and put together a "game plan" to work together over the coming months.

The North has remained publicly defiant about the intensified sanctions.

It says the US inspired the UN to condemn its April rocket launch, which it described as a peaceful attempt to put an experimental satellite in orbit.

The US and other nations saw it as a disguised test of a Taepodong missile, which is theoretically capable of reaching Alaska.

On Saturday its official news agency reported comments by its ceremonial head of state Kim Yong-Nam at the Non-Aligned Movement summit in Egypt earlier this week.

Kim "asserted that if such acts of the US are allowed to go on, the DPRK (North Korea) would be totally deprived of the legitimate right to use space," the official agency said.

"The DPRK can never accept dialogue or negotiations minus the principle of respect for sovereignty and equal sovereignty," Kim Yong-Nam was quoted as saying.

"The prevailing situation compelled the DPRK government to take decisive steps to bolster up its nuclear deterrence."